1. Field of the Invention
The invention relates to a method of determining a parameter indicative of the progress of an extracorporeal blood treatment, in particular a purification treatment whose purpose is to alleviate renal insufficiency, such as haemodialysis or haemodiafiltration.
2. Description of the Related Art
It will be recalled that haemodialysis consists in making a patient's blood and a treatment liquid approximately isotonic with blood, flow, one on each side of the semipermeable membrane of an exchanger, so that, during the diffusive transfer which is established across the membrane in the case of substances having different concentrations on either side of the membrane, the impurities in the blood (urea, creatinine, etc.) migrate from the blood into the treatment liquid. The electrolyte concentration of the treatment liquid is also generally chosen so as to correct for the electrolyte concentration of the patients blood.
In treatment by haemodiafiltration, a convective transfer by ultrafiltration, resulting from a positive pressure difference created between the blood side and the treatment-liquid side of the membrane, is added to the diffusive transfer obtained by dialysis.
It is of the utmost interest to be able to determine, throughout a treatment session, one or more parameters indicative of the progress of the treatment so as to be able, where appropriate, to modify the treatment conditions that were fixed initially for the purpose of a defined therapeutic objective.
The parameters, the knowledge of which makes it possible to follow the progress of the treatment, i.e. also to assess the suitability of the initially fixed treatment conditions to the therapeutic objective, are, in particular, the concentration in the blood of a given solute (for example, sodium) or the actual dialysance or the actual clearance of the exchanger for such and such a solute (the dialysance and the clearance representing the purification efficiency of the exchanger) or the dialysis dose administered after a treatment time t, which, according to the work of Sargent and Gotch, may be likened to the dimensionless ratio Kt/V, where K is the actual clearance for urea, t the elapsed treatment time and V the volume of distribution of urea, i.e. the total volume of water in the patient (Gotch F. A. and Sargent S. A., "A mechanistic analysis of the National Cooperative Dialysis Study (NCDS)", Kidney Int. 1985, Vol. 28, pp, 526-34).
These parameters all have the same problem in respect of their determination, which is of requiring precise knowledge about a physical or chemical characteristic of the blood, whereas this characteristic cannot in practice be obtained by direct measurement on a specimen for therapeutic, prophylactic and financial reasons: firstly, it is out of the question to take, from a patient who is often anaemic, multiple specimens which would be necessary in order to monitor the effectiveness of the treatment during its execution; furthermore, given the risks associated with handling specimens of blood which may possibly be contaminated, the general tendency is to avoid such handling operations; finally, laboratory analysis of a specimen of blood is both expensive and relatively lengthy, this being incompatible with the desired objective.
Document EP 0,658,352 describes a method for the in vivo determination of the haemodlalysis parameters which does not require taking measurements on the blood. This method, the implementation of which requires means for controlling the ion concentration in the treatment liquid and means for measuring the sodium concentration in the treatment liquid or its conductivity, comprises the steps of:
making at least a first (d1) and a second (d2) treatment liquid, having a characteristic (Cd) associated with at least one of the parameters (Cb, D, K, Kt,V) indicative of the treatment, flow in succession through the exchanger, the value of the characteristic in the first liquid (d1) upstream of the exchanger being different from the value of the characteristic (Cd) in the second liquid (d2) upstream of the exchanger; PA1 measuring, in each of the first (d1) and second (d2) treatment liquids, two values (Cd1in, Cd1out; Cd2in, Cd2out) of the characteristic (Cd), respectively upstream and downstream of the exchanger; PA1 making a third treatment liquid (d3) flow through the exchanger while the characteristic (Cd) of the second liquid (d2) has not achieved a stable value downstream of the exchanger, the value of the characteristic (Cd) in the third liquid (d3) upstream of the exchanger being different from the value of the characteristic (Cd) in the second liquid (d2) upstream of the exchanger; PA1 measuring two values (Cd3in, Cd3out) of the characteristic (Cd) in the third liquid (d3) respectively upstream and downstream of the exchanger; and PA1 calculating at least one value of at least one parameter (Cb, D, K, Kt/V) indicative of the progress of the treatment from the measured values of the characteristic (Cd) in the first (d1), second (d2) and third (d3) treatment liquids. PA1 making a treatment liquid flow through the exchanger, this treatment liquid having a characteristic (Cd) which has an approximately constant nominal value (Cd0in) upstream of the exchanger; PA1 varying the value of the characteristic (Cd) upstream of the exchanger for a time tc-ta, at the end of which the characteristic (Cd) is returned to its nominal value (Cd0in) upstream of the exchanger; PA1 measuring and storing in memory a plurality of values adopted by the characteristic (Cd) of the treatment liquid downstream of the exchanger in response to the variation in the value of this characteristic (Cd) caused upstream of the exchanger; PA1 determining the area (Sout) of a downstream perturbation region bounded by: PA1 setting the characteristic, over a first defined time interval, to a first defined set value above (or below) the nominal value (Cd0in), and PA1 setting the characteristic, over a second time interval, to a second set value below (or above) the nominal value (Cd0in). PA1 giving the parameters of the variation a defined value, so that the area of the upstream perturbation region Sin is constant when the predetermined parameters of the variation are complied with; PA1 determining and storing in memory the area of the upstream perturbation region Sin for a variation having the fixed parameters.